Lost Chick - Help

I lost my Silver Duckwing Phoenix 5 week chick about an hour ago. He was very lethargic when I went to check on him after work so I put him in my lap and frantically started to google. Shortly after, it seems like he threw up and then went into spasms and died. The only answer I can find is coccidocis or however it's spelled, but they've been on medicated feed their whole life. I have 3 more chicks and I need to know how to keep this from happening to them.